The nicotinic acetylcholine receptors (nAChRs) are members of a superfamily of ligand-gated ion channels that mediate fast signal transmission at synapses. The ion channel is formed from the assembly of a membrane protein oligomer (a pentamer) that binds the neurotransmitter, acetylcholine, its natural ligand. The nAChR also binds agonists, such as nicotine, and nicotinic antagonists, such as mecamylamine. The binding of two molecules of acetylcholine or nicotine to the alpha subunits of the receptor induces a conformational change, stabilizing the receptor's open-state, which allows the flux of ions (e.g., sodium, calcium, potassium) across the cell membrane. An influx may cause membrane depolarization and the activation of voltage-gated ion channels for sodium and calcium, resulting in the exocytotic release of neurotransmitters and hormones from vesicular stores. The overall conductance as well as the relative conductances of various ions depend on the subunit composition of the receptor (Lindstrom et al., Ann. NY Acad. Sci. [1995] 757:100-116; McGehee and Role, Annu. Rev. Physiol. [1995] 57:521-546). Nicotinic receptors are found in both muscle and neuronal tissues and are therefore broadly classified as either muscle-type or neuronal nicotinic AChRs.
There are multiple types of nAChRs in the brain associated with synaptic function, signal processing or cell survival. The therapeutic targeting of nicotinic receptors in the brain requires the identification of drugs that may be selective for their ability to activate or inhibit a limited range of these receptor subtypes.
Brain nicotinic receptor systems have long been associated with addiction. Recently, it has been shown that nicotinic receptor systems may be involved with Tourette's syndrome (Silver, A. A. et al., J. Am. Acad. Child Adolesc. Psychiatry [1996] 35:1631-1636; Sanberg, P. R. et al., Lancet [1998] 352:705-706) and schizophrenia (Adler, L. E. et al., Biol. Psychiatry [1992] 32:607-616, Adler, L. E. et al. Am. J. Psychiatry [1993] 150:1856-1861; Leonard, S. et al, Soc. Neurosci. Abstr. [1993] 19:837; Freedman, R. et al., Harvard Rev. Psychiatry [1994] 2:179-192) and that nicotinic drugs may also have applications as analgesics and for the treatment of Alzheimer's disease (Williams, M. et al., Drug News Perspect. [1994] 7:205-223; Arneric, S. P. et al., Psychopharmacology: The Fourth Generation of Progress [1995], pp. 95-110). With these newly defined therapeutic endpoints, the challenge becomes understanding how best to target nicotinic drugs to the receptor systems of the brain.
The pharmacology of neuronal nicotinic receptors, however, is very complex. With a gene family that includes at least nine different a subunits (designated α2-α10) that in some cases may function as homooligomers (α7-α10) or alternatively combine with different neuronal β subunits (β2-β4), there is a great potential for structural diversity just on the level of the basic pentamer receptor subunit combinations (Papke, R. L. Prog. Neurobiol. [1993] 41:509-531). Multiple receptor subtypes are commonly found on single neurons, and single tissues have multiple neuronal cell types that differ in the function of their nicotinic receptors (Mulle, C. et al., J. Neurosci. [1991] 11:2588-2597).
One approach for sorting out significant elements in this complex system is to study cloned receptor subunits in defined combinations. The co-expression of α4 and β2 subunits represents one receptor subunit combination of particular interest, as the primary high-affinity nicotinic receptor of the brain is composed of these subunits (Whiting, P. J. and J. M. Lindstrom, Biochemistry [1986] 25:2082-2093; Flores, C. M. et al., Mol Pharmacol. [1992] 41:31-37). Receptors containing the α3 subunit are also likely to be found in the brain but predominate in the peripheral nervous system (Halvorsen, S. W. and D. K. Berg, J. Neurosci. [1990] 10:1711-1718). The properties of both brain and ganglionic nicotinic ACh receptor (nAChR) can be modified by the co-assembly with the nonessential α5 subunit (Conroy, W. G. et al., Neuron. [1992] 9:679-691; Wang, F. et al., J. Biol. Chem. [1996] 271:17656-17665; Gerzanich, V. et al., J. Pharmacol. Exp. Ther. [1998] 286:311-320). Another important type of brain nicotinic receptor subtype are those that bind α-bungarotoxin with high affinity. These receptors correspond to the α7 subunit gene products, which form homomeric receptors with high calcium permeability and fast desensitization to high concentrations of agonist.
Furthermore, nicotinic receptor subunits exhibit considerable promiscuity in their ability to coassemble to form functional channels in various expression systems. Therefore, it is possible that alternative subunit combinations may result under certain conditions (e.g., tissue injury, chronic drug exposure). By recombinant expression study with specific combinations of receptor subunits, the relative efficacy and potency of available nicotinic agonists and antagonists have been defined (Brioni et al., Adv. Pharmacol. [1997] 37:153-214; Holladay et al., J. Med. Chem. [1997] 40:4169-4194; Lloyd and Williams, J. Pharmacol. [2000] 292:461-467). Therefore, there would seem an opportunity for developing drugs that have greatly increased selectivity with respect to receptor subtype specificity. Unfortunately, subtype selective agonists and antagonists have been only slowly forthcoming.
Many of the experimental new nicotinic agents being considered for clinical development, including GTS-21, ABT-418, ABT-089, and SIB-1553A (Meyer et al., Brain Res. [1997] 768:49-56; Papke et al., Br. J. Pharmacol. [1997] 120:429-438; Sullivan et al., J. Pharmacol. Exp. Ther. [1997] 283:235-246; Lloyd et al., Life Sci. [1998] 62:1601-1606), have very mixed profiles of agonist and antagonist activity, meaning that each agent has both excitatory and inhibitory effects on nicotinic receptors in the brain. This mixed pharmacological profile is also observed in the prototypic cholinergic ion channel agonist, nicotine. Unfortunately, because of their mixed agonist/antagonist profiles, the toxic side effects produced by these agents hinder their development as therapeutic drugs.
Accordingly, considerable need exists for agents which selectively target nicotine acetylcholine receptors and individual receptor subtypes (i.e., receptor subunit combinations), and which avoid the toxic side effects associated with the administration of compounds that are mixed activators and inhibitors of nicotine acetylcholine receptors.